Underwater oil storage tank and method of submerging same

ABSTRACT

A submerged tank for location on the sea bottom has upwardly opening ballast chambers. The ballast chambers have a bottom member that rests directly on the sea bottom. The storage tank is open to the entrance of water into the lower portion of the storage compartment of the tank from the surrounding waters whereby pressure within the tank and in the surrounding water is substantially equal. In a preferred embodiment the tank is of annular shape with a central ballast chamber and an outer ballast chamber surrounding the storage compartment. The tank is filled with a liquid of lower density than sea water to give it sufficient buoyancy to be floated to the desired site and is sunk at the site by displacement of low density liquid with sea water. After location of the tank on the bottom, the ballast compartments are filled with ballast to secure the tank on the sea bottom.

1 1 Dec. 24, 11974 States atent Westling 1 UNDERWATER OIL STORAGE TANKAND FOREIGN PATENTS OR APPLICATIONS METHOD OF SUBMERGING SAME [75]Inventor: Randolph E. Westling, Houston,

Tex.

Primary ExaminerW. C. Reynolds Assistant Examiner-Alex Grosz [73]Assignee: Gulf Oil Corporation, Pittsburgh,

[22] Filed: Aug. 3, 1972 ABSTRACT [21] A submerged tank for location onthe sea bottom has Appl. No.: 277,698'

upwardly opening ballast chambers. The ballast cham- Related ApplicationData bers have a bottom member that rests directly on the Continuationof Ser. No. 152,715, June 14, 1971, sea bottom. The storage tank is opento the entrance abandoned.

of water into the lower portion of the storage compartment of the tankfrom the surrounding waters whereby pressure within the tank and in thesurrounding water is substantially equal. In a preferred embodiment thetank is of annular shape with a central ballast chamber and an outerballast chamber surrounding the storage compartment.

mnmm 5/ 9 0 M8 2 1. 1 ,D 5B .5 0 0 ,T 6 67 5 .l 14 m2 H 0" M N Tm C .r.nua e S L h l WM .w UhF ,UHN 555 References Cited I UNITED STATESPATENTS The tank is filled with a liquid of lower density than sea waterto giveit sufficient buoyancy to be floated 61/465 to the desired siteand is sunk at the site by 114/5 T displacement of low density liquidwith sea water. 61/-5 After location of the tank on the bottom, theballast 1 5 compartments are filled with ballast to secure the tank61/465 0n the sea bottom. 114/5 T 7 Claims, 4 Drawing Figures 2,622,40412/1952 Rice................. 3,327,667 6/1967 Manning 3,605,774 9/1971Launay et al.... 3,630,161 12/1971 Ge0rgii............. 3,703,20711/1972 Horton..... 3,740,956 6/1973 Guy 3,766,583 10/1973 PATENTEI][E024 I974 sum 2 or 3 UNDERWATER OIL STORAGE TANK AND METHOD OFSUBMERGING SAW This is a continuation of application Ser. No. 152,715,filed June 14, 1971, now abandoned.

This invention relates to the storage of oil and more particularly to asubmerged storage tank for offshore locations and a method of loweringthe storage tank onto the sea bottom.

A large part of the oil consumed in the world is now produced atoffshore wells. It can be expected that the percentage of oil producedfrom offshore locations will increase in the future because many of themost promising onshore locations have already been thoroughly explored.As offshore exploration and drilling continues, wells will be drilled atincreasing distances from the. shore and in water of increasing depth.Delivery of oil produced by the wells will, therefore, become moredifficult. A pipeline from each production platform to the shore isprohibitively expensive, and production platforms are not large enoughfor supporting tanks of adequatev volume on the production platforms.Moreover as the depth of water in which wells are drilled increases, thecost of production platforms will become excessive and wellheadassemblies will be located close to the sea bottom.

One method of storing oil that has been suggested is in floating storagetanks, which may be of either rigid or flexible construction. Such tankswould be exposed to violent wave action during storms and consequentlycould cause serious oil spills; The most promising method of handlingproduction for offshore wellsin deep water and at substantial distancesfrom shore appears to be to deliver the oil into submerged tanks restingon the sea floor. The tanks will necessarily be of large capacity topermit continued operation of the wells between loadings of tankerswhich will take the oil from thesubmerged storage tanks for delivery toonshore sites. I v

Because mostoil is substantially less dense than sea water, difficultyis experienced in holding storage tanks securely on bottom when the tankfilled with oil. One method that has been suggested is to sink thestorage vessel onto the bottom and secure the vessel on bottom withpiling. Filling the storage tank with oil and then displacing the oilfrom the tank with sea water causes an alternate upward and downwardforce on the piling.

I Continued reversals of the direction of the force on the piling tendsto loosen the piling.

It has been suggested that storage tanks for use under water be made ofconcrete and have sufficient weight as constructed to overcome thebuoyant force exerted by oil when the-tank is filled with oil. Tanks oflarge volumetric capacity and heavy enough to overcome the volume of theair. Such close control of the buoyancy of an air-filled tank isdifficult.

This invention resides in an underwater storage tank open at its bottomand upwardly opening confined spaces outside of thestorage compartmentfor receivingballast after the tank is on the sea bottom. The storagecompartment is open to sea water at its lower end to equalize thepressure inside and outside of the storage tank. The storage tank isfilled with a liquid having a lower density than sea water to provide astructure with a positive buoyancy that can be floated to the offshoresite. Part of the low density liquid is displaced from the storage tankwith sea water to sink the structure onto the sea bottom. The upwardlyopening confined spaces are then filled with ballast material,preferably sea bottom solids,-in an amount adequate to hold the tanksecurely on bottom when the tank is filled with oil. A delivery linefrom the upper end of the storage tank extends to the water surface andis supported by a suitable float.

In the drawings:

FIG. 1 is a diagrammatic vertical sectional view of a preferred storagetankof annular shape, constructed in accordance with this invention.

FIG. 2 is a diagrammatic vertical sectional view of a verticalcylindrical storage tank.

FIG. 3 is a perspective view of abank of horizontal cylindrical tanksutilizing this invention.

FIG. 4 is a diagrammatic view of a tank utilizing this invention beinglowered while stabilized by cables from barges.

Referring to FIG. 1, an underwater tank constructed in accordance withthis invention and indicated generally by reference numeral 10 isillustrated resting on the sea bottom 12 well below the water surface14. Storage tank 10 isof vertical annular configuration with a verticalcylindrical outer wall 16 concentrically surrounding a verticalcylindrical inner wall 18. A top 20 extends from the upper end of innerwall 18 to the upper end of outer wall 16 to closethe upper end ofstorage tank 10 and combine with the inner and outer walls to define astorage compartment 22 within the tank. Suitable bracing 24 across thetop of the tank aids in supporting top 20.

A circular'bottom member 26 secured to the lower end of the inner wall18 closes the lower end of a centrally located upwardly openingcylindrical bin 22 adapted to receive ballast, as. hereinafterdescribed, to

aid in holding the storage tank on the sea bottom 12. Encircling andspaced from outer wall 16 is a cylindrical retainer wall 30. The spacingof retainer wall 30 from outer wall 16 will depend on the size of thestorage tank 10 and ballast bin 28. For example, in a storage tankhaving a capacity of approximately 200,000 barrels the diameter of thetank could be 200 feet, the diameter of the bin 28 40 feet, and theheightof the tank 40 feet. The space between outer wall 16 and retainerwall 30 would then be 10 feet to provide a ballast chamber 32 betweenthe retainer wall 30 and the outer wall 16 which would overcome thebuoyancy'of the tank when the tank is filled with oil. The height ofretainer wall 30 is preferably the same as storage tank 10; however, theretainer wall 30 can be lower in which event it may be necessary toincrease the distance between the retainer wall and the outer wall toprovide a ballast bin 32 of sufficient volume. An annular bottom plate34 extends from the lower end of outer wall 16 to the lower end ofretainer wall 30 and is secured to those walls to close the lower end ofthe ballast chamber 32. It is preferred that suitable bracing 36 beprovided between retainer wall 30 and outer wall 16 to stiffen thestorage tank structure. The lower end of the storage compartment 22 isopen, but it can be provided with framework indicated by referencenumeral 37 to increase the rigidity of the storage tank.

In the storage tank illustrated in FIG. 1 of the drawings, the lower endof ballast chamber 32 and ballast bin 28 have been filled to a depth,for example one foot, with concrete; and the outer wall 16, inner wall18, and top 20 have been coated with concrete to a thickness, forexample, of six inches. The concrete coating reduces the buoyancy of thestorage tank, stiffens the structure, and reduces corrosion of the steelwalls of the tank.

Storage tank is provided with an oil inlet line 38, preferably openinginto the upper part of storage compartment 22. Oil inlet line 38 may beconnected directly to wells completed on the sea bottom or may beconnected to production facilities supported on a production platformlocated either above or under the water surface. A sea water exchangeline 40 extends through retainer wall 30, ballast chamber 32, and outerwall 16 and opens into the lower part of the storage compartment 22. Seawater exchange line 40 is open to provide equalization of pressurebetween the inside and outside of the storage tank and allow entry ofsea water to displace oil from the storage tank or outflow of sea wateras oil is delivered through inlet line 38 into the storage compartment22.

An oil outlet 42 opens through top and is connected to a flexible oildelivery line 44. A suitable bridle 47 limits movement of delivery line44. The upper end of delivery line 44 is supported at the water surface14 by a float 46 and is provided with valves as required for control ofthe flow of oil delivery from the storage tank 10.

Storage tank 10 is constructed, preferably of light weight steel, at agraving dock. Because there is substantially no pressure differentialbetween the inside and outside of the storage tank at any time duringthe floating, sinking, or use of the tank, the tank can be constructedof steel approximately [/4 inch thick. The lower end of the ballast bin28 and the ballast chamber 32 are partially filled with concrete; andthe outer wall 16, inner wall 18, and top 20 are coated with concrete asdescribed above to increase the rigidity and reduce the buoyancy of thetank. The addition of concrete into the lower ends of the ballastcompartment 32 and ballast bin 28 and'the coating of the outer surfaceswith concrete are preferred but are not essential to this invention.Tank 10 is then floated by delivering water into the graving dock. Afterthe tank is lifted from the bottom of the graving dock, an oil,preferably an oil having an API gravity of 30 (density of 7.300 poundsper gallon) or less, is used to displace air from the storage tank andthereby reduce the buoyancy of the tank. In a steel tank of annularshape having a capacity of approximately 200,000 barrels as describedhereinbefore, filling the tank with an oil having a gravity of 30 APIoil will result in a net buoyancy of about 4,700 tons when the tank isfloating in sea water. Filling the lower portion of bin 28 and chamber32 and coating the walls and roof of the tank adds approximately 3,443tons of concrete, which have a net weight of about 2,220 tons in seawater, and reduces the net buoyancy to approximately 2,480 tons.

The floating tank is towed to the site and sunk to the desired locationby allowing sea water to displace oil from the tank into a barge ortanker to reduce the buoyancy of the tank until it will slowly sink ontothe sea bottom. Oil inlet line 38 and delivery line 44 are closed whilethe tank is towed to the offshore location, and oil inlet line remainsclosed as the tank is sunk onto the sea bottom. Guide and stabilityassistance, during lowering, may be obtained by cables from a floatingvessel to the tank during the lowering period.

Because the oil is incompressible, there is no effective change in thedensity of the oil as the tank sinks and hence no change in its negativebuoyancy. In contrast, if the tank is filled with air to supplysufficient buoyancy to float the tank to the offshore site, as isrequired if the tank has sufficient weight as constructed to remain onthe bottom when filled with oil, the air is compressed as the tank sinksand the negative buoyancy of the tank rapidly increases. It is thennecessary to deliver compressed air into the tank at a controlled rateto prevent the tanks plunging to the bottom. Moreover, open-bottomedtanks filled with air suffer a large and rapid loss of buoyancy if theyshould tilt and burp while being floated to the site or sunk on bottom.

Although oil has been described as a liquid supplying the buoyancynecessary to float the tank, other liquids can be used. For example, ifsea water can be introduced into the graving dock to float the tank,fresh water can be introduced into the storage compartment 22. Thedifference in density of the fresh water and the sea water will providesufficient buoyancy to float the tank.

' I Cable assist may be used in lowering the tank to the sea bottom,depending upon the size and shapeof the tank. This may be by pull-downcables from floating vessels through tiedown pulleys anchored on the seabottom. In this case a slight positive buoyancy would be maintained inthe tank until it was on or near bottom at which time a negativebuoyancy would be attained by change in the oil-water ratio in the tank.The advantage of an unchanging buoyancy of the tank while it is loweredonto the bottom resulting from using a low density liquid to supply thedesired buoyancy is also advantageous in the cable assist method oflowering.

Referring to FIG.. 4, storage tank 10 is shown diagrammatically withcables attached at intervals around the retainer wall 30. Cables 110extend downwardly to anchors 112 which can be of the type described inmy US. Pat. No. 3,431,879. The cables 110 pass through a sheave 114 onthe upper end of the anchors and extend upwardly to winches 116 mountedon barges 118. ln sinking the storage tank onto the bottom 12, theanchors l12are lowered from barges 118 by means of cables 110 to rest onthe sea bottom 12 around the desired site of the storage tank 10.Anchors 112 are then filled with bottom material if they are of I thetype disclosed in US. Pat. No. 3,431,879. The free ends of the cablesare attached to the wall 30 of the tank 10. Oil is displaced from thetank until the tank to be recovered or to secure the cables to theanchors if the anchors are to be left in place to further secure thetank against movement by bottom current.

In the apparatus illustrated in FIG. 2 of the drawings, a submergedstorage tank, indicated generally by reference numeral 50, differingfrom storage tank only in the omission of the centrally located ballastbin that is provided in storage tank 110, has a vertical cylindricalouter wall 52 closed at its upper end by conical top 54. A retainer wall56 encircles the outer wall 52 and is spaced from that wall a distanceproviding a ballast chamber 58 having a volume that will hold anadequate weight of ballast material to hold the tank on bottom whenfilled with oil. The lower end of the ballast chamber58 is closed by abottom member 60 joining the lower ends of retainer wall 56 and outerwall 52. Botsecured to the inner surface of wall 52, stiffens the tank.The upper end of oildelivery line 68 is connected to a float. 72 at thewater surface 74. Submerged tank 50 is illustrated with concrete coatinginstalled similarly to the concrete on storage tank 10. The manner ofconstruction, transportation to the site, and sinking of storage tank 50at the offshore site is the same for tank 50 as tank 10.

Referring to FIG. 3 'of the drawings, a battery of three horizontalcylindrical tanks 80 is shown located on the sea bottom 82, asubstantial distance below the water surface 84. Horizontal tanks 80 arearranged with their longitudinal axes parallel and are spaced apart toprovide a space between the outer walls of the middle tank and the wallsof the outer tanks. A bottom plate 86 closes the lower end of the spacebetween the tanks 80, and end plates 88 close the ends of the spacesbetween the tanks to provide upwardly opening chambers 90 between thetanks for the reception of ballast material. A sea water exchange line92 serves as a header with lateral lines 94 opening into the lower partof each of the tanks 80. Similarly, an oil inlet line 96 extends acrossone end of the tanks and is provided with lateral inlet lines 98 openinginto the upper portion of each of tanks 80. Inlet line 96 is connectedwith wells or other production equipment for delivery of oil into thetanks 80. Oil outlet line 100 opens from the upper portion of each oftanks 80 near the centerthereof and is connected through a manifold 102to an oil delivery line 104. The upper end of oil delivery line 104 issupported on the water surface by a float 106.

As in the other embodiment of this invention, tanks 80 can beconstructed and assembled at a graving dock with the bottom plate 86 andend plates 88 secured to the tanks 80 tanks 80 filled with a liquid oflower density than sea water, the graving dock filled with sea water,and the assembled tanks floated from the dock and towed to the offshoresite. It will be necessary, of course, to provide suitable closures foroil inlet line 96 and oil delivery line 104 to prevent displacement ofoil from the tanks as the tanks are moved to the offshore location.Tanks 80 are sunk onto bottom by opening oil delivery line 104 to allowsea water to displace the liquid of low density from the tanks until thetanks have a slightly negative buoyancy. After tanks have been sunk ontothe sea bottom, the upwardly opening space defined by the walls of tanks80 and the end plates 88 are filled with suitable ballast material,preferably sea bottom material, in an amount that will hold the tankssecurely on the sea bottom even when the tanks are completely filledwith oil.

Although this invention has been described for tanks constructed oflight weight steel, it is not limited to such material of construction.Similar savings can be made in tanks constructed of concrete. lf thisinvention is employed in the construction of concrete storage tanks forsea bottom sites, it is necessary only to provide concrete thicknessadequate to supply the strength required by the storage tank, not of athickness that will give the tanks sufficient weight to hold them onbottom. Since it is contemplated that the tank will be of open bottomconstruction, i.e., means are provided for free flow sea water into thelower portion of the tank as the volume of oil storage in the tank ischanged, relatively light weight construction is required regardless ofwhether the tank is constructed of concrete or steel. The installationof ballast adequate to hold the tank on bottom when the tank is filledwith oil after the tank has been located on bottom permits a very largereduction in the weight of the storage tank and greatly facilitates itsconstruction and handling in moving to the offshore site.

An advantage of the storage tanks of this invention is that the bottomsof the ballast chambers or ballast bins rest directly on the sea bottom.Such structure is important in further reducing the steel required forthe tanks by eliminating the structural supports that would be requiredif the ballast were placed on surfaces, such as theroof of the storagecompartment, spaced above the sea bottom.

As an example of the weight saving of the storage tank structure that ismade possible by this invention, assume a vertical annular tank of thetype illustrated in FIG. l and having 200,000 barrel capacity isconstructed having a outer diameter of 200 feet. The weight of 200,000barrels of 30 API oil is 61,280,000 lbs., and the weight of the same.volume of sea water is 71,860,000 lbs.; therefore the oil supplies abuoyant force of approximately 5,300 tons. The weight of the steelstructure illustrated in FIG. 1 is about 500 net tons plus tons ofbracing, which gives a net buoyancy of the tank when filled with 30 APIoil of 4,700 tons. Assuming a 37 percent safety factor to preventmovement of the tank by under water currents, an in-water weight of7,250 tons would be required. Since the net weight of the steel is 600tons, ballasting material having a net weight in sea water of 6,650 tonsis required. Adding the ballast material before the tank is on bottommakes it necessary to handle a massive structure while floating the tankto the desired location and sinking in onto the sea bottom. Moreover, itwould be necessary to use gas to supply the buoyancy required to floatthe tank to the site.

Coating the sides and top of the tank with 6 inches of concrete andfilling the bin 28 and chamber 32 with concrete to a depth of 1 footwould add 3,443 tons of concrete to the steel tank structure. The netweight in sea water of 3,443 tons of concrete would be 2,220 tons. Thus,even after the addition of the concrete, a

storage tank filled with oil would have a buoyancy of 2,470 tons.Filling the remaining space in the bin 28 and chamber 32 with sea bottommaterial having a net weight of 36 lbs. per cubic foot in sea waterwould add 5,520 tons to the weight of the structure and provide anegative buoyancy of 2,780 tons which would be more than adequate toinsure stability of the storage tank on the bottom.

As an additional example of the weight savings made possible if twohorizontal cylindrical tanks 48 feet in diameter and 326 feet long areconstructed of concrete to provide 200,000 barrels of capacity, it wouldbe necessary for the tanks to have a wall thickness of 12 inches ofconcrete with top and bottom slabs 6 inches thick joining it to thetanks to provide a structure with sufficient weight to be stable onbottom when filled with 30 API oil. Utilizing this invention, thehorizontal cylindrical tanks could be sunk onto the sea bottom whenfilled with liquid, only a portion of which is oil,

such as a 30 API petroleum oil. The addition of ballast after the tanksare sunk on bottom allows the tanks to have only the wall thicknessrequired to give the tanks the desired strength.

I claim:

1. A submerged storage tank for installation on the sea bottom atoffshore locations to store oil having a lower density than sea watercomprising a vertical outer wall enclosing a storage compartment, a topmember extending across the top of the storage compartment and connectedto the outer wall to close the upper end of the storage compartment,said storage compartment being open at its lower end, a retainer wallsurrounding the vertical wall and spaced therefrom to cooperate with thevertical wall in defining an upwardly opening ballast chamber betweenthe retainer wall and the outer wall, a bottom member connected to thelower end of the vertical wall and to the retainer wall closing thelower end of the ballast chamber, said bottom member positioned andadapted to rest on the sea bottom when the tank is sunk onto theoffshore site, a sea water exchange line extending from outside of theretainer wall into the storage compartment and opening into the lowerpart of the storage compartment, an oil inlet line opening into theupper part of the storage compartment, and an oil delivery lineextending from the top of the storage compartment upwardly to the watersurface.

2. Apparatus as set forth in claim 1 in which the tank is of annularshape having a vertical inner wall within the outer wall, said topmember extending from the inner wall to the outer wall to leave thespace within the vertical inner wall opening upwardly to form a centralballast chamber, and a central bottom member connected to the lower endof the inner wall and extending across the lower end of the centralballast chamber.

3. Apparatus as set forth in claim 2 in which the inner and outer wallsare cylindrical and the retainer wall surrounding the outer wall is ofsubstantially the same height as the outer wall.

4. A submerged storage tank as set forth in claim 1 in which thevertical outer wall, the top member, the retainer wall and the bottommember are constructed of steel and a layer of concrete extends over theupper surface of the bottom member, the outer surface of the verticalouter wall and the top member to increase the rigidity of the structure.

5. In a method of installing a submerged oil storage tank at an offshorelocation, said tank having upwardly opening ballast compartments outsideof the storage compartment, the lower end of the storage compartmentbeing open, the improvement comprising floating the tank on water,filling the storage compartment with oil to provide a buoyancy adequateto float the tank, floating the tank to the desired offshore site,displacing a portion of the oil from the tank with sea water to give.the tank a slight negative buoyancy to sink the tank onto the seabottom, and filling the ballast chambers with ballast material after thetank is on bottom and in an amount adequate to hold the tank on bottomwhen the tank is filled with oil.

6. In a method of installing a submerged oil storage tank at an offshorelocation, said tank having upwardly opening ballast compartments outsideof the storage compartment, the lower end of the storage compartmentbeing open, the improvement comprising floating the tank on water,filling the storage compartment with oil to provide a buoyancy adequateto float the tank, floating the tank to the desired offshore site,attaching cables at intervals around the periphery of the storage tankand running the cables to barges surrounding the storage tank,displacing oil from the tank with sea water until the tank has a slightnegative buoyancy, sinking the tank onto the sea bottom while guidingthe tank with the cables to maintain it in a horizontal position,filling the ballast chambers with ballast material after the tank is onbottom, and disconnecting the cables from the tank.

7. In a method of installing a submerged oil storage tank at an offshorelocation, said tank having upwardly opening ballast compartments outsideof the storage compartment, the lower end of the storage compart mentbeing open, the improvement comprising floating the tank on water,filling the storage compartment with oil to provide a buoyancy adequateto float the tank, floating the tank to the desired offshore site,sinking a plurality of anchors onto the sea bottom surrounding thedesired site of the tank, running cables from the tank through sheaveson the anchors and to barges surrounding the tank, displacing oil fromthe tank with sea water until the tank has a slight positive buoyancy,taking up on the cables to pull the storage tank downwardly onto the seabottom, and filling the ballast tank on bottom.

1. A submerged storage tank for installation on the sea bottom atoffshore locations to store oil having a lower density than sea watercomprising a vertical outer wall enclosing a storage compartment, a topmember extending across the top of the storage compartment and connectedto the outer wall to close the upper end of the storage compartment,said storage compartment being open at its lower end, a retainer wallsurrounding the vertical wall and spaced therefrom to cooperate with thevertical wall in defining an upwardly opening ballast chamber betweenthe retainer wall and the outer wall, a bottom member connected to thelower end of the vertical wall and to the retainer wall closing thelower end of the ballast chamber, said bottom member positioned andadapted to rest on the sea bottom when the tank is sunk onto theoffshore site, a sea water exchange line extending from outside of theretainer wall into the storage compartment and opening into the lowerpart of the storage compartment, an oil inlet line opening into theupper part of the storage compartment, and an oil delivery lineextending from the top of the storage compartment upwardly to the watersurface.
 2. Apparatus as set forth in claim 1 in which the tank is ofannular shape having a vertical inner wall within the outer wall, saidtop member extending from the inner wall to the outer wall to leave thespace within the vertical inner wall opening upwardly to form a centralballast chamber, and a central bottom member connected to the lower endof the inner wall and extending across the lower end of the centralballast chamber.
 3. Apparatus as set forth in claim 2 in which the innerand outer walls are cylindrical and the retainer wall surrounding theouter wall is of substantially the same height as the outer wall.
 4. Asubmerged storage tank as set forth in claim 1 in which the verticalouter wall, the top member, the retainer wall and the bottom member areconstructed of steel and a layer of concrete extends over the uppersurface of the bottom member, the outer surface of the vertical outerwall and the top member to increase the rigidity of the structure.
 5. Ina method of installing a submerged oil storage tank at an offshorelocation, said tank having upwardly opening ballast compartments outsideof the storage compartment, the lower end of the storage compartmentbeing open, the improvement comprising floating the tank on water,filling the storage compartment with oil to provide a buoyancy adequateto float the tank, floating the tank to the desired offshore site,displacing a portion of the oil from the tank with sea water to give thetank a slight negative buoyancy to sink the tank onto the sea bottom,and filling the ballast chambers with ballast material after the tank ison bottom and in an amount adequate to hold the tank on bottom when thetank is filled with oil.
 6. In a method of installing a submerged oilstorage tank at an offshore location, said tank having upwardly openingballast compartments outside of the storage compartment, the lower endof the storage compartment being open, the improvement comprisingfloating the tank on water, filling the storage compartment with oil toprovide a buoyancy adequate to float the tank, floating the tank to thedesired offshore site, attaching cables at intervals around theperiphery of the storage tank and running the cables to bargessurrounding the storage tank, displacing oil from the tank with seawater until the tank has a slight negative buoyancy, sinking the tankonto the sea bottom while guiding the tank with the cables to maintainit in a horizontal position, filling the ballast chambers with ballastmaterial after the tank is on bottom, and disconnecting the cables fromthe tank.
 7. In a method of installing a submerged oil storage tank atan offshore locatioN, said tank having upwardly opening ballastcompartments outside of the storage compartment, the lower end of thestorage compartment being open, the improvement comprising floating thetank on water, filling the storage compartment with oil to provide abuoyancy adequate to float the tank, floating the tank to the desiredoffshore site, sinking a plurality of anchors onto the sea bottomsurrounding the desired site of the tank, running cables from the tankthrough sheaves on the anchors and to barges surrounding the tank,displacing oil from the tank with sea water until the tank has a slightpositive buoyancy, taking up on the cables to pull the storage tankdownwardly onto the sea bottom, and filling the ballast chamber withballast material to anchor the storage tank on bottom.